The present invention relates to an image forming apparatus forming an image on an output medium, and to an image outputting method.
In an image forming apparatus which can output a color image by overlapping monochrome images by using an intermediate transfer medium, an effective image holding length which can hold images corresponding to the largest output medium which can be outputted is given to the intermediate transfer medium. Namely, when the intermediate transfer medium is cylindrical, the circumference thereof is defined to be able to hold the aforementioned largest image, and when the intermediate transfer medium is belt-shaped, the overall length thereof is defined to be able to hold the aforementioned largest image.
Therefore, in an image forming apparatus in which an intermediate transfer medium, to which a length corresponding to the long side of an A3 sized paper is given as the effective image holding length of the intermediate transfer medium, is built in, due to an A4 sized paper being conveyed such that the direction perpendicular to the direction of conveying the paper is the long side direction of the (A4) paper (hereinafter, simply called A4 portrait), a method for simultaneously outputting images of two A4 portrait on the intermediate medium has been put into practice.
Incidentally, when images of two A4 portrait are simultaneously outputted on the intermediate transfer medium, there is the problem that the images cannot be outputted until the image data for two A4 portrait (for two pages) of all of the monochrome images which can be overlapped on the intermediate transfer medium is obtained.
Namely, the compiling time required for compiling image data into output-image data varies, according to the complexity of the data, such as the density and size of the image data per page. Therefore, in an image outputting method in which toner images corresponding to output-image data are sequentially transferred onto an intermediate transfer medium, because the image forming operation starts at the point in time when the total output-image data of the monochrome images which can be overlapped on the intermediate transfer medium has been obtained, there is the problem that the number of image formations (number of outputs) per unit time is small.